The present invention relates to miniature inductors and pertains particularly to improved monolithic inductors and a method of manufacturing the same.
Miniature inductors are widely used in radio frequency electric circuits. The inductors are made in two basic configurations: wire wound and monolithic. Wire-wound inductors are made with a wire wound on a dielectric or a ferrite core, or they can be made free-standing, provided a wire of sufficient thickness is used to ensure stability. There are several types of monolithic inductors: multilayer ceramic and ferrite, a single-layer spiral, and helical.
Wire-wound inductors, especially the ones wound on dielectric cores or free-standing, generally have high Q-factor values, but are characterized by high cost. It is also relatively complicated to provide an inductor having an exact inductance value, due to the fact that the locations of wire termination points are usually fixed on a core, so fractional wire turns are sometimes not possible. One solution to this problem has been to select cores of different diameters for various inductance values, but this leads further to increased costs due to the need to adjust winding and handling machinery and the need for expanded core inventories.
In addition, the repeatability of the winding process, especially in cases where the inductance value is relatively low and only a few wire turns are required, is limited by the same need to attach the ends of the wire to the fixed locations on the core.
The multi-layer monolithic inductors are made of a series of stacked ceramic or ferrite layers each having a conductive trace deposited on its surface. The traces on adjacent ceramic layers are electrically interconnected. Multi-layer inductors are generally less expensive than wire-wound but have significantly lower Q-values, which limits their usefulness in some applications.
A spiral-type of inductor consists of a flat ceramic substrate on which single layer flat spiral metal pattern is defined. They have the same limitations as the multi-layer inductors.
A helical-type monolithic inductor consists of a substrate forming an elongated ceramic or ferrite core. The substrate is covered with one or more metal layers which are then etched or cut in a helical fashion, either mechanically or with a laser beam. The cut defines a helical winding similar to a wire coil. The ends of the core usually have metal caps in electrical contact with the conducting layer. The metal caps customarily have a solderable coating defining terminals to facilitate soldering of the inductor to a printed circuit board.
I have discovered through extensive testing that the end caps on the current helical monolithic inductors act as shortened windings. These shortened windings introduce parasitic losses which lead to diminished Q-values and lower inductance values. I have also discovered that without continuous metallization at the end caps significantly increased Q-values, and inductance values are obtained.
It is desirable to reduce cost of electrical components such as inductors without sacrificing performance of the systems they are made part of. It is also desirable to provide inductors as close to the stated nominal value as possible, with minimal process variations. This is known in the art as "high tolerance". Having inductors with high tolerance is desirable since it may obviate the use of tunable components, such as inductors and capacitors in end products.